The present invention relates to a dot line printer, and more particularly to a printing hammer assembly of the dot line printer capable of performing high speed printing. Throughout the specification and claims, the expression "front", "rear", "above", "below" and "laterally" are used herein to define the various parts when the print hammer assembly is disposed in an orientation in which it is intended to be used. For example, "front" implies a printing pin side, and "laterally" implies a direction of an array of a printing hammers.
A printing hammer assembly of a conventional dot line printer is shown in FIGS. 1 and 2. A comb yoke 18 is mounted on a base 14, and a permanent magnet 17 is interposed between the comb yoke 18 and the base 14. The comb yoke 18 has front surface defining a plurality of attraction surfaces having rectangular shape in cross-section. Further, an electromagnetic coil 19 is wound over a front portion, adjacent to a pole portion of the comb yoke 18. A leaf spring 11 has a base end fixed to a front face of the base 14 by a screw 16, and a front yoke 15 is positioned over the leaf spring 11, and is secured to the front face of the base 14 by the screw 16. The front yoke constitutes a part of a magnetic circuit. The leaf spring 11 has free end portion formed with a plurality of arm portions defining hammer springs arrayed side by side at a predetermined pitch in a lateral direction (shuttling direction of the hammer bank assembly). Each of printing pins 12 projects frontwardly from each of the arm portions of the leaf spring 11, and plungers 13 also protrude frontwardly from the arm portion at a position adjacent the printing pins. The arm and the printing pin constitute a printing hammer. The plungers 13 have cylindrical configuration, and corresponding bores are formed in the front yoke 15 to allow the plungers 13 to pass therethrough. Because of the magnetic force of the permanent magnet 17, rear faces of the plungers 13 are attracted to the pole portion provided at the front end portion of the comb yoke 18 with accompanying flex of the arm portions in order to maintain non-printing position of the pins. Further, because of the energization of the electromagnetic coil 19, the free end portion of the leaf spring are released from the pole portion to perform printing. Thus, spring-charged dot printing hammer is provided.
In order to perform high speed printing, two arrays of printing pins have been proposed. That is, the printing hammer assembly has an upper array of printing pins, and a lower array of printing pins, and totally 200 numbers or less of pins are provided with a pitch of the neighboring pins being 0.15 inches in order to print about 400 lines of Chinese characters per a minute. Further, if the printing speed is to be further enhanced to print about 600 lines of Chinese characters per minute, about 300 numbers of printing pins are required with the pin pitch of 0.1 inches. However, with high density arrangement of the printing pins, disadvantages may be caused.